1747
Communications to the Editor
COMMUNICATIONS TO THE EDITOR
Chemically Activated I4CH3CF3from Cross Combination of I4CH3 with CF3. It. Collisional Energy Transfer to Fluorinated Ethanes Publication costs assisted by the U.S. Air force Office of Scientific Research
Sir: The substance CH3CF3 has been the subject of several recent chemical activation It has been demonstrated that under unfavorable circumstances the experimental decomposition/stabilization (D/S) product ratios are subject to systematic errors caused by secondary reactions of CF3 radicals with the unimolecular decomposition product CH2=CF2.1p2p4 Two different procedures have been suggested for controlling this problem. Neely and Carmichael varied the photolysis duration in the CH~COCHS-CF~COCF~ system and obtained extrapolated (D/S) ratios corresponding to zero ketone conversion.2 In this laboratory a “protective scavenging” technique has been developed that is based upon the controlled chemical suppression of the CF3 c~ncentration.~,~ The quantitative significance of spurious CF3 reactions for collisional energy transfer applications has not previously been established. In the present work we wish to communicate energy transfer results for activated 14CH3CF3generated via radical combination in ketone cophotolysis experiments? This technique provides 102 f 2 kcal mol-l of initial excitation:315 14CH3+ CF3
(14CH3CF3)t
(1)
At internal energies above 70 kcal mol-’ 14CH3CF3can decompose via HF elimination:
2 ‘4CHpCFz + H F
(14CH3CF3)t
(2)
in which k, denotes the phenomenological, nonequilibrium unimolecular rate constant. The alternate fate for activated 14CH3CF3 involves collisional stabilization through efficient “strong” or cascading “weak” energy transfer encounters with the host gas (M):
+
(14CH3CF3)t M
14CH3CF3+ M
(3)
In this work (D/S) is given by the product ratio (14CH2=CF2/14CH3CF3). Assuming unit collisional deactivation efficiency, (D/S) can be equated to ( k , / o ) , the ratio of the unimolecular rate constant to the bimolecular collision frequency. Since w is proportional to pressure, k, then follows from the experimental quantity [(D/S)P].7 The high-pressure unimolecular rate constant (k,”) has been defined as the pressure corresponding to unit (D/S) ratio as determined from linear regression analysis of the (D/S) vs. (1/P) data. The pressure range for this calculation corresponded to (D/S) ratios of 1.5 or l e s ~ . It~ .is~important to The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
TABLE I: Calculated Energy Transfer Increment Results @E) 9 Bath gasa
kcal mol-‘ collision“
*
4.0 0.7 6.3 i 0.7 CF3CFH2 4.0 i 0.7 CF~HCFZH 5.2 i 1.0 CF3CH3 4.0 i 0.7 CF3CH3b 6.3 i 0.7 CFZHCFH, 4.0 0.7 CFZHCH, 4.0 0.7 CFH2CH3 4.0 2.0 a All systems protected with 6 mole % CHz=CFz unless specified otherwise. Unprotected pure CH3CF3. C2F6
C2F5H
* * *
recognize that the unit deactivation description is not strictly applicable for 14CH3CF3,even for polyatomic colliders.3~~ The characterization of collisional energy transfer based upon pressure fall off data measured in the cascade deactivation region requires the availability of theoretical unimolecular rate constants together with a suitable computer algorithm to simulate the cascade.8 Our computed rate constants were obtained with the aid of the Universal RRKM program of Hase and Bunker? together with reverse dissociation and four-center 1334 HF elimination critical configuration models.1° A standard stepladder algorithm was used to determine average collisional energy transfer increments ( AE) through computer simulation of the low-pressure fall off data.3*7The two-component stepladder calculation properly accounted for the fact that roughly 6% of the deactivating collisions took place with the CH2=CF2 additive.11-13 In order to facilitate direct comparison of the present C2F6 (AE)value with that reported previously? identical computer algorithms and critical configuration models were employed. The (AE) results reported in Table I were obtained by matching the curvatures of experimental and theoretical (ka/kam)vs. (S/D) p10ts.l~ Our experimental pressure fall off results for CH3CF3 bath gas are shown in Figure 1 both in the presence and absence of the CH2=CF2 additive. The marked curvature exhibited by these fall off plots for (D/S) ratios larger than about 1.5 is indicative of a cascading deactivation mechanism. The principal point of the present report has to do with the systematic decrease in (D/S) ratios observed in Figure 1 for the unprotected relative to the protected CH3CF3 system. This effect leads to a significant decrease in the curvature and, therefore, to systematic errors in the calculated (AE)values. Figure 2 depicts the influence of the spurious 1 4 C H ~ C Flosses 2 upon the ( AE) calculation.
1748
Communications to the Editor io
harnvalue is in good agreement with the 23 Torr 298OK result reported by Setser et al. However, our protected CzF6 system yielded a ( AE)of 4.0 f 0.7, in poor agreement with the reported value of 7.0 kcal mol-’ colli~ion-~.~ These differences are real and cannot be ascribed to variations in calculational procedure or to experimental accuracy limitations. Additional observations follow from Table I. Nearly all of the protected systems yielded ( AE) values of 4.0 f 0.7 kcal mol-’ collision-l, which is rather small for “strong” colliders in the context of the current literature,3J? Finally, the host substance CH~CFB apparently does not exhibit unusually efficient energy transfer behavior toward activated 14CH3CF3.
0
2
0
002
006 01 014 018 Reciprocal Pressure (Torr-’)
Figure 1. (D/S) ratios vs. reciprocal pressure for protected and unprotected CH3CFs host gas: (A)6 mol % CH2=CF2, protected system; (0)unprotected system. I .9
Acknowledgments. The authors acknowledge assistance with computer programs and discussions with Professors B. S. Rabinovitch, D. W. Setser, D. L. Bunker, and W. Hase. Dr. C. F. McKnight assisted with the early unprotected CH3CF3 experiments. Financial support has been provided by the U.S.Air Force Office of Scientific Research.15 One of us (J.W.R.) also received support from a John Simon Guggenheim Fellowship (1972-1973).
References and Notes (1) R. D. ales and E. Whittle, Trans. Faraday SOC.,61, 1425 (1965). (2) 6.D. Neely and H. Carmlchaei, J. Phys. Chem., 77, 307 (1973). (3) H. W. Chang, N. L. Cralg, and D. W. Setser, J. Phys. Chem., 76, 954 (1972). (4) R. R. PettiJohn,G. W. Mutch, and J. W. Root, J. Phys. Chem., in press. (5)G. W. Mutch, Ph.D. Dissertation, University of California, Davis, 1973; Available from University Microfilms as Dissertatlon No. 74-8530. (6) The photolysis mixture consisted of a 9.64:1 CF3COCF3:CH3COCH3 blend in whlch the acetone component was 14C iabeied in the 1,3 posltions at a specific radioactivity of 8.53 i 0.01 mCi mmol-l. The samples were phototyzed at 307 1°K using 310-nm ultraviolet radiation. Additional details have been glven in ref 4 and 5. (7) (a) P. J. Robinson and K. A. Holbrook, “Unimolecular Reactions”, Why. New York, N.Y., 1972. p 164. (b) Note that w = PZ where P denotes
1.7
.I! I 5 c 0
a
iI
09 00
10
20 30 (S/D) Ratio
40
Figure 2. (k,/ka”) ratios vs. (S/D) ratios for protected and unprotected CH3CF3 host gas. Plot points as in Figure 1, Theoretical stepladder curves given as (A€) values: A, 4.0; B, 6.3;C, 8.6; D, 10.9 kcal mol-’ collision-’, respectively.
These stepladder fits have included the usual empirical scaling p r o ~ e d u r e , ~ and , ~ the CH3CF3 experimental (ka/ ham)data, for example, have been scaled by a constant factor of 1.15. This procedure facilitates matching the curvatures of calculated vs. experimental stepladder curves. The present protected vs. unprotected CH3CF3 systems yielded significantly different values of 4.0 f 0.7 vs. 6.3 f 0.7 kcal mol-l collision-l, respectively. We conclude that the unprotected system yielded a serious (ca. 60%) positive systematic error in ( AE). Related experiments have demonstrated similar errors with other unprotected fluorinated e t h a n e ~For . ~ ~this ~ reason the CH2=CF2 additive technique has been routinely utilized. Our C2F6 bath gas results can be directly compared with literature data. The 21.6 f 1.4 Torr 307OK experimental
(a)
The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
the pressure (Torr) and Z the bimolecular collision number (Torri1 sec-I). (8) D. W. Setser, “Unimolecular Reactions of Polyatomic Molecules. Radicals, and Ions,” In MTP International Review of Science, Physical Chemistry, Volume 9, 1972, p. I f f . (9) W. Hase and D. L. Bunker, private communication (10) These critical configuration models were taken from ref 3. The reaction path degeneracies for the decomposition and reverse dissocietlon models were 6.0 and 1.0, respectively. (1 1) The mixed collision diameters used in the energy transfer calculations were obtalned from critical data (ref 12): CHz==CF2, 4.74; CFH2CH3, 4.81; CFeHCHa, 4.92; CF~HCFHZ,4.96; CH~CFS, 4.98; CFZHCF~H, 5.03; CF3CFH2, 5.03; CF3CFzH, 5.04 and CzF6, 5.04 A. Critical constants for several of the fluorinated ethanes were measured in this laboratory by Mr. F. E. Little. (12) J. W. Root, Ph.D. Dissertation. University of Kansas, 1964; Available from University Microfilms as Dissertation No. 65-7004. (13) The previous study (ref 4) showed that (A€) for CHz=CFz was smaller than the values for any of the fluorinated ethanes. In the present calculations for mixtures containing 6 mol % CHz==CFz, the (A€) values obtained for the principal bath gas components were Insensitive to the a s s u y (A€) for CH2==CFz over the range 1-3 kcal mol-’ colllsion(14) The stepladder algorithm calculated and plotted (k.lkam) ratios with kadefined as the ka value corresponding to 200 Torr pressure. (15) This research has been supported by U.S. Air Force Offlce of Scientific Research Contract No. AF-AFOSR-73-2460.
.
Department of Chemistry University of California Davis, California 956 16 ReceivedFebruary 26, 1975
Rlchard R. Peltijohn George W. Mutch John W. Root.
